shot count
 

any way to tell if a glock has been fired a lot. say 200 to 500 rounds. i see a lot advertised as on shot at factory. does shooting gun leave a mark on barrel.

Shooting the Glock will often enough leave a few marks on the barrel however I have seen many pistols that look as new and shoot several hundred rounds.
There is no worry about a Glock shooting several thousand rounds. Thats about what it takes to lap in the trigger (naturally)